"Black minnow jig" would likely be a jighead with a minnow shaped plastic body that is black in color.
Many people think dirty water means they need to put on a bright lure, but actually you're better off using a darker color lure. Having one that has two colors (dark/light) can also work well, as it provides a contrast ... as well as being visible in varying degrees of "dirty".
There really isn't any "best" anything, when it comes to lures. If there were, there'd only be that one lure for sale. Any lure or jig will work at specific or certain times. How you present the lure is the #1 factor, provided there are any Crappie where you're presenting that lure.
Presenting the lure means : at what depth, & at what speed (+ what action the bait has or you create). Crappie are generally looking forward & up when feeding. They're not going to chase a bait very far, or very fast ... they're more into ambushing or being sneeky/stealthy like a ninja.
Whenever you "figure this Crappie thing out" ... you let me know, asap, because I've been trying for nearly 60yrs and haven't done it yet.
But, seriously .... casting & swimming a jig/plastic is done a little differently than casting & retrieving a Roadrunner.
I like to cast the RR and just simply reel it back in at a "moderately steady" pace with rod tip pointing down at almost a 45deg angle. The hookset is merely a sweep of the rod while I keep on reeling. The strike is usually just a sudden weight on the end of the line.
When I'm casting & swimming a jig/plastic, I reel it back with a "very slow retrieve", and my rod tip is pointing up at almost a 45deg angle. The hookset is a fast lifting up and back of the rod, then resume reeling. The strike can be a single "tic" in the line (line jumps), or the line moves off to one side or the other, or the line suddenly goes slack and you know the lure is not on the bottom.
Next time you start catching small ones (esp. this time of year), first go deeper and if that don't work, go shallower. If that don't work, go to another spot. If the fish are moving into the shallows to spawn, they're going to be of nearly the same size or year class. Usually the older/bigger fish will take over the spawning banks first, leaving the smaller/younger ones to wait their turn. And if you are catching the males shallow, and they're displaying darker than usual black markings, but not catching any of the bigger/fatter females .... then the females may be underneath or behind you. Move farther away from the bank and fish at nearly the same depth as you were catching the males, and fish around any submerged timber or over any channel dropoff that's close to the area of the bank where the males were caught. The females will be "staging" out there, and only making occasional moves into the shallow bedding areas ... depositing a portion of their eggs into any one nest, move back to the staging area & rest, then go back in and drop another portion of their eggs into another nest. They will repeat this until they have no more eggs or the conditions get messed up ... in which case they may wait & see if things improve & try again, or they may just give up trying and absorb their eggs as nourishment.
... cp